Fagot and colleague Robert Cook, a professor of psychology at Tufts University in Boston, Massachusetts, report the findings today in the early online edition of the journal Proceedings of the National Academy of Sciences.

According to the researchers, the findings suggest that expansion of memory may have driven the evolution of intelligence.

"One way to think about it is to ask what an easy upgrade to a computer is," Cook said. "One of the easiest is to add a large hard disk."

Hard disks contain computer memory.

"You don't have to buy a new monitor or a new, faster [processor]. Just a bigger hard drive increases the performance of the machine. It makes more information available," he added.